The present invention relates to a power supply apparatus for supplying power to a load and, more particularly, to a power supply apparatus in which a plurality of DC power supplies may be added in order to supply power to a single load.
FIG. 6 is a circuit diagram showing the arrangement of a conventional power supply apparatus of this type.
This power supply apparatus comprises many DC power supply devices 21A, 21B, . . , 21N (N is a positive integer) for supplying power to a load R. These DC power supply devices 21A, 21B, . . . , 21N have the same arrangement.
The schematic arrangement of the DC power supply device 21A will be exemplified.
The DC power supply device 21A uses a DC voltage from a power supply E as a switching voltage based on switching operation of a switching transistor Q1, and supplies this switching voltage to the primary winding of a transformer T1. A voltage induced on the secondary winding is rectified by diodes D1 and D2 as rectifiers, and smoothened by a smoothing coil L1 and smoothing capacitor C1. The smoothened voltage is output as an output voltage V0 via a diode D3. This output voltage V0 is supplied to the load R.
The DC power supply device 21A also has a pulse width modulation (PWM) controller 10 for controlling switching operation of the switching transistor Q1. The PWM controller 10 is controlled by an internal output voltage control circuit 11 to adjust the output voltage V0.
The internal output voltage control circuit 11 changes the ON width of the switching transistor Q1 by the PWM controller 10 under the control of an output current from a photo-coupler PC1 by an output control element IC12. The internal output voltage control circuit 11 has the photo-coupler PC1 made up of a combination of a light-emitting diode PD1 and photo-transistor QP connected to the positive electrode of the smoothing capacitor C1, a resistor R13 connected between the photo-coupler PC1 and ground, and the output control element IC12.
The internal output voltage control circuit 11 further comprises a series circuit of a voltage adjustment rheostat VR11 and resistors R11 and R12 series-connected between the anode of the light-emitting diode PD1 in the photo-coupler PC1 and ground, and a coupling capacitor C12 connected between the node of the resistors R11 and R12 and the node of the resistor R13 and output control element IC12. Note that the node of the resistors R11 and R12 is connected to the gate electrode of the output control element IC12.
The remaining DC power supply devices 21B, . . . , 21N have the same arrangement as the DC power supply device 21A.
In the conventional power supply apparatus shown in FIG. 6, the output voltage adjustment rheostats VR11 incorporated in the DC power supply devices 21A, 21B, . . . , 21N independently adjust their output voltages V0. Hence, if the output voltages from the DC power supply devices 21A, 21B, . . . , 21N are different in the conventional power supply apparatus, their output currents are also different.
To prevent this, the conventional power supply apparatus requires a current balancing circuit in addition to the internal output voltage control circuit 11 when two or more of the DC power supply devices 21A, 21B, . . . , 21N parallelly operate. This complicates the whole arrangement of the power supply circuit.
Moreover, each DC power supply device requires an internal output voltage control circuit 11. The occurrence probability of an overvoltage fault of the output voltage V0 or the like becomes twice the number of DC power supply devices, and the possibility of the debilitation of the entire power supply apparatus increases.